1. Field of the Invention
The present invention relates to analysis of heavy metals in a silicon wafer used for fabrication of semiconductor devices such as ICs and LSIs. More particularly, the present invention relates to a method and apparatus for detecting, with high sensitivity, heavy-metal impurities contained in a wafer bulk in the course of a process for manufacturing wafers or a process for fabricating semiconductor devices.
2. Description of the Related Art
During a process for manufacturing wafers or a process for fabricating semiconductor devices, heavy-metal impurities such as iron, copper, and nickel sometimes become contaminated into a wafer at the surface or in the bulk thereof. It is known that if such heavy-metal impurities exist in a region where semiconductor devices will operate, the electric characteristics of the semiconductor devices are degraded, and thus the operations thereof are hindered. Heavy-metal impurities existing at the surface of a wafer do not pose a great problem, because they can be directly analyzed and can be removed through cleaning. By contrast, heavy-metal impurities contained into the bulk of a wafer are difficult to analyze directly and are also difficult to remove. In addition, such heavy-metal impurities directly affect the characteristics of semiconductor devices. Therefore, highly sensitive detection of heavy-metal impurities within the bulk of a wafer is considerably important.
In order to analyze such heavy-metal impurities within the bulk of a wafer, there have conventionally been used chemical analysis, secondary-ion mass spectroscopy analysis, and the like.
Among them, chemical analysis is basically limited to analysis of samples in liquid form. Therefore, when a solid sample such as a semiconductor wafer is evaluated, for example, the surface of the wafer is chemically treated through use of HF solution, and the solution used for the treatment is collected and then subjected to atomic absorption spectroscopic analysis or ICP (Inductively Coupled Plasma) emission spectroscopic analysis in order to specify elements. Especially, in the field of semiconductors, there is used vapor phase decomposition/flameless atomic absorption spectroscopic analysis or vapor phase decomposition/ICP mass spectroscopic analysis. These methods have a disadvantage that the reliability of analyzed values are greatly affected by contaminants introduced into a sample during a pre-treatment process thereof. Further, much time and labor are needed to perform a pre-treatment process, and a person that performs the pre-treating must have a certain level of operational skill.
Also, there has been reported a technique in which a silicon wafer is subjected to heat treatment in order to aggregate impurities existing within the bulk to the surface or to the vicinity thereof, and the above-described chemical analysis is performed in order to analyze impurities within the bulk. However, this technique has a problem of contamination caused by the heat treatment.
Meanwhile, the secondary-ion mass spectroscopy analysis is a method for locally analyzing elements at high sensitivity and is used for analysis of impurities contained in a semiconductor in minute quantities. In this method, a beam of ions (primary ions) such as O.sub.2.sup.+, Cs.sup.+, or Ga.sup.+ having an energy of a few hundreds of eV to a few tens of keV or neutral particles such as those of Ar are radiated onto the surface of a sample. As a result, atoms at the surface of the sample are emitted into vacuum by means of sputtering. Among the sputtered particles, ionized particles (secondary ions) are extracted through application of an electric field and are subjected to mass spectroscopic analysis through use of a magnetic field and an RF electric field. Thus, the types and concentrations of elements contained on the surface of the sample are determined. This method is mainly used for analysis with respect to the depth direction of a sample, such as measurement of the dopant profile within a semiconductor material, and for analysis of behavior of impurities. However, an apparatus used in this method is expensive and maintenance of the apparatus is complicated and cumbersome because super-high vacuum is required.
Also, impurity analysis has been performed through use of total reflection x-ray fluorescence analysis. The total reflection x-ray fluorescence analysis is a method for non-destructive analysis of elements locally existing at the "surface" or at the "vicinity of the surface" of a sample in minute quantities. Therefore, by its very nature this method is inapplicable to detection of impurities within the bulk of a sample.
Although analysis of intra-bulk heavy metals through chemical analysis has a high detection sensitivity, this analysis requires a pre-treatment, which would contaminate a sample. In addition, since the analysis requires a destructive process such as dissolution of a sample, it is not simple and is time consuming.
The method in which analysis is performed after heat treatment is applied to a silicon wafer in order to aggregate heavy-metal impurities to the surface or to the vicinity thereof has a possibility of inducing secondary contamination caused by the heat treatment, so that true analysis values are difficult to obtain.
The analysis of intra-bulk heavy-metal impurities by use of the secondary-ion mass spectroscopy analysis has a disadvantage that the sensitivity changes depending on the types of elements, and therefore the setting and adjustment of measurement conditions are not easy. This method is also destructive.
The total reflection x-ray fluorescence analysis enables simple and non-destructive analysis of heavy-metal impurities contained in a silicon wafer. However, this method only analyzes the surface of a wafer and the vicinity thereof and cannot analyze the inside of the bulk. That is, the total reflection x-ray fluorescence analysis is applicable to a depth of only 100 Angstroms, and in ordinary analysis, evaluation is performed to a depth of 20-30 Angstroms.
If for some reason contamination caused by Cu, among heavy metals, occurs at the surface of a wafer, such contamination easily diffuses into the inside of the bulk. In this case, the analysis methods for analyzing or evaluating the surface of a wafer and the vicinity thereof cannot detect contamination even though the inside of the bulk is contaminated, or detects such contamination as being lower than the actual level. As described above, contamination caused by Cu is difficult to detect through an ordinary surface analysis. Therefore, even when a test for heavy metals is performed, contamination caused by heavy metals is overlooked, so that defectives may be generated is a subsequent device-fabrication process.
In general, when a method for analyzing the surface of a wafer is used, impurities existing at the surface of the wafer cannot be detected unless the concentration of impurities is greater than that corresponding to the detection limit of the analysis equipment. That is, even when contamination exists within the bulk, such contamination cannot be detected by the surface analysis alone. Especially, in the case of contamination caused by Cu, such contamination is sometimes not detected at the surface of a wafer even when the bulk of the wafer is contaminated to a level of 10.sup.15 atoms/cm.sup.3.